Electrolyte for the polishing of metal surfaces and method of use



Patented Nov. 23, 1943,

ELEOI'ROLYTE FOB m POLISHING OF METAL SURFACEBANDMETHOD OF USE Charles L. Fault, Columbus, Ohio, allignor to Memorial Battelle corporation of Ohio v No Drawing.

Millie, Columbus, 01110, a

Application November 23, 1938, Serial No. 241,975

I '2 Claims. (ol. 204-140) This invention relates to an electrolyte for the polishing of metal surfaces and more particularly to an electrolytic solution of high mineral acid concentration for use in the anodic treatment of metals to impart thereto a lustrous, mirror-like polish.

It has heretofore been common practice to polish metals by a mechanical operation, either on automatic polishing and bufllng machines, or manually, or by a combination of automatic and manual operations. 'In addition to the considerable cost of mechanical polishing and bufling operations, particularly where the final buffing operations have to be performed manually, there are some drawbacks in the mechanical polishing of metals which are inherent because of the properties of the metals themselves. Some metals, such as stainless steels, for instance, are relatively poor conductors of heat, in consequence of which the high speeds of the polishing or bufiing wheels tend to burn the surfaces thereof. This greatly limits the speed at which the polishing operations may be carried out.

Furthermore, in the mechanical polishing of metals, the surfaces exhibit a tendency to "pile, with the result that the polished surfaces so produced are cold worked and strained. Such surfaces are characterized by the presence of a layer of amorphous or pseudoamorphous materials. Also, in'any commercial polishing process, the surface will be found to be marked by microscopic scratches.

Mechanical polishing has the further objection that it involves a considerable amount of comparatively highly paid labor. In the case of stainless steel, for example, mechanical polishing is one of the most expensive steps in the finishing operations, so that polished stainless steels have heretofore sold for a price which is very high in comparison with the material cost. The cost of mechanical polishing is also a major item in the manufacture of chromium plated articles, particularly articles of irregular shape that must first be polished, then nickel plated and finally bufled before the application of the chromium plate.

While electrolytic methods for the treatment of metals and alloys to clean or etch the surfaces of the metals are well known, such methods have brighter than that obtainable by the usual pickling processes.

Similarly, although the Blaut et a1. Patent 2,115,005, dated April 26, 1938. p rts to describe an electrochemical cleaningprocess that will produce a "burnished" surface, I have been unable to obtain by following the procedure therein disclosed such a highly lustrous polish as is readily produced by the use of the electrolytic solution of my present invention.v I

I have found that a superior polishing effect may be produced more readily and at a lower cost through the use of an electrolytic solution consisting essentially of a mixture of sulfuric and phosphoric acids provided that the combined concentration of the acid is at least 50% by weight of I the solution. Such an electrolytic solution is particularly-applicable in the polishing of stainless steel, nickel, Monel metal, German silver and plain carbon steels, as well as various other metals. in the case of plain carbon steels,

low alloy steels and in general non-austenitic steels, I have found it particularly advantageous to add a minor proportion of chromic acid to the 5 mixture of sulfuric and phosphoric acid.

It is therefore an important object of my invention to provide an electrolytic solution for use in the anodic treatment of metals to impart thereto a highly lustrous surface directly and without requiring any subsequent mechanical builing or polishirg operation.

It is a further important object of my invention to provide an electrolytic solution particularly adapted for use in the anodic treatment of metals to produce thereon surfaces that are highly lustrous and free from the scratchesand piled" layers characteristic of mechanically polished surfaces of metals such as stainless steel.

It is a further important object of this invention to provide an electrolytic solution for use in the polishing of metals that will enable the polishing to be carried out at a cost considerably less than that entailed by mechanical polishing operations, while at the same time producing surfaces having relatively superior characteristics to those obtainable by mechanical polishing or bufllng operations.

Other and further important objects of this invention will become apparent from the following description and appended claims.

The method to which this invention pertains involves making the metal to be polished an anode in an electrolytic bath of suitable composition and passing a current of sufllclent density and for a. sufficient length of time to produce a high lustre or polish on the metal. By employing the electrolytic solution of my present invention, results are obtained that transcend a mere electrolytic cleaning process, in that there is produced was. highly lustrous or polished surface. This high polish and lustre is an important feature of my invention and one that sharply distinguishes it from prior art finishes produced in the electrolytic cleaning of metals. Y

The electrolytic solution that I have found to possess the most general applicability comprises a mixture of sulfuric and phosphoric acid in such proportions that the combined acid concentration is at least 50% by weight of the solution, the balance being mainly water. Sulfuric and phosphoric acids, particularly ortho phosphoric acid, are eminently suitable for the purpose, mainly because of their stability and the ease with which they produce satisfactory polishes.

' While it is possible to produce polishes with mixtures of sulfuric and phosphoric acids containing as much as 50% water, I prefer to keep the water content of-the electrolytic bath relatively low, since I have found, in general, that baths containing lower proportions of water may be operated at lower current densities, and therefore at a lower cost, to give satisfactory polishes. I therefore prefer to have the total acid content in excess of 50% by weight, but not over 90%, the balance being mainly water except where minor proportions of other addition materials, such as chromic acid, are employed to improve the polishing effect of the bath toward certain metals. The relative proportions of sulfuric to phosphoric acid may, in general, be varied from' 5% of sulfuric and 85% of phosphoric acid to 5% of phosphoric acid and 85%0f sulfuric acid, but the phosphoric acid content is preferably in excess of 50% of the total bath composition. Unless otherwise specified, all percentages referred to herein will be understood to represent percentages by weight, rather than by volume. Also, references to sulfuric and phosphoric acids should be understood to mean the respective.

acids themselves as represented by their chemical formulae and not as some commercial strength of acids..

In order to obtain the desired results in a reasonable length of time, I prefer to use relatively high current densities in my electropolishing method. With the preferred mixtures of sulfuric and phosphoric acids, the process can be operated with anode current .densities varying from 50 to 1000 amperes per sq. ft. It will be understood that even lower current densities may be employed, but longe'r times of treatment will be required and the process may for that reason he uneconomical. Higher current densities than indicated by the foregoing range may also be used, but in that case the higher cost of electrical energy offsets the saving in time. The length of time to effect the desired results depends upon the magnitude of the current densities employed, and to some extent upon the particular metal to be polished and the character of its surface initially. Rough surfaces, of course, require a longer time to polish than relatively smooth ones.

The mechanism by which the highly polished and lustrous surfaces characteristic of my invention are produced is not well understood, but is undoubtedly associated with the presence of a polarizing film over the surface of the metal during the process of anodic attack. The nature of this film is such that selectiv attack of the various phases present in the metal, or alloy, is minimized. Anodic dissolution apparently takes place at a relatively high rate at high anodic po larization value. lution of the metal under these conditions is to level the crystal surface of the metal to produce EXAMPLE 1 1 Bath composition, per cent by weight Range Preferred -20% 15% sulfuric acid. 63-67% 63% ortho phosphoric acid. Balance 22% water.

With the above bath, excellent polishes are obtained on 18-8, 25-12 chromium-nickel stainless steels, straight chromium stainless steels containing from 12 to 18% chromium, or even higher, nickel, Monel metal, German silver, Chromel and plain carbon steels, when these are made the anodes therein at current densities of 50 amperes per sq. ft. for one hour more or less, depending upon the original condition of the surface. At higher current densities less time will be required. For instance, I have found that this bath composition produces a mirror polish on nickel when the nickel is made the anode therein at current densities of 125 to 250 amperes per sq. ft. for from 4 to 12 minutes at temperatures in the neighborhood of 50 C. The same bath will also produce an excellent polish on a gray nickel eleetroplate at current densities of 125 to 250 amperes per sq. ft. within a period of from 2 to 6 minutes. In eiectropolishing Monel, Nichrome and Chromel alloys, similar conditions may be used, but I prefer to operate at an anode current density of about 100 amperes per sq. ft. for a period of 20 minutes at a. temperature of about 40 C.

EXAMPLE 2 Bath composition, per cent by weight Per cent Sulfuric acid 60 Ortho phosphoric acid 30 Water 10 A bath of this composition produces excellent polishes on 18-8 and 24-12 chromium-nickel 1 stainless steels at current densities of 250 amperes per sq. ft. or more, and on nickel and nickel alloys, such as Monel metal, German silver, and the like, at current densities of 250 amperes per sq. ft. The temperatures empoyed may be in the neighborhood of 50 0., although that is not critical.

It will be apparent from the bath compositions above given that considerable latitude is permissible in the respective percentages of sulfuric and phosphoric acids, provided that the combined acid content is over 50%, and preferably over 75%. In general, it is preferable to keep the sulfuric acid content within the range of 15 to 60% and the ortho phosphoric acids within the range of 15 to the combined acid content being between 50 and 90% by weight of the solution.

When electropolishing plain carbon steels and molybdenum steels, I find it preferable to include chromic acid (CIOa) in the electrolytic The result of the anodic dissosolution. The proportions of sulfuric and phosphoricacids, however, are still kept within the broad ranges above given. The following examples will serve to illustrate typical electrolytic solutions containing sulfuric, phosphoric and chromic acids.

EXAMPLE 3 Bath composition, per cent by weight Per cent Sulfuric acid 15 Ortho phosphoric acid 63 Chromic acid 10 Balance water.

EXAMPLE 4 Per cent Sulfuric acid 20 Ortho phosphoric acid 67 Chromic acid 2 Balance water.

EXAMPLE 5 Per cent Sulfuric acid 40 Ortho phosphoric acid 44 Chromic acid 6 Balance water.

In the polishing of plain carbon steels, molybdenum steels and the like, using the foregoing compositions, I have found that the total acid should be over 60% in order to produce mirrorlike polishes. The addition of chromic acid greatly increases the polishing effect of the sulfuric-phosphoric acid mixture and is preferably added in amounts up to 20%, the percent of chromic acid, however, being in general not over 10% and therefore less than the percentage of either sulfuric or phosphoric acid.

EXAMPLE 6 The following bath composition has been found satisfactory in the electropolishing of copper:

Per cent Sulfuric acid Ortho phosphoric acid Chromic acid Balance water.

EXAMPLE 7 I In the electropolishing of low carbon steels, the chromic acid may be entirely omitted and good results obtained by the use of a bath of the following composition:

4 Per cent Sulfuric acid 40 Ortho phosphoric acid 49.5

Balance water.

It will thus be apparent that I have provided a novel electrolytic solution for use in producing a polish on various metals, including stainless steels, nickel and its alloys and plain carbon steels. Mymethod of utilizing the bath compositions described above obviates the disadvantages of mechanical polishing operations and enables the production of polished metal articles having surfaces that are free from mechanical strains, dragging and piling, and that are better than those made by mechanical polishing methods.

otherwise than necessitated by the scope of the appended claims.

I claim as my invention:

1. An electrolytic solution for use in the anodic polishing of metals selected from the groupconsisting of iron and its alloys, nickel and its alloys and German silver, consisting essentially of from 5 to 85% of sulfuric acid and from 5 to 85% of ortho phosphoric acid, the combined acid content being at least 50% but not over 90% by weight of said solution and the balance being mainly water.

2. An electrolytic solution for use in the anodic polishing of metals selected from the group .consisting of iron and its alloys, nickel and its alloys and German silver, consisting initially of from 15 to 60% of sulfuric acid and from 15 to 70% of ortho phosphoric acid, the combined acid content being between 50 and 90% by weight of said solution and the balance being mainly water.

3. An electrolytic solution for use in the anodic polishing of metals selected from the group consisting of iron and its alloys, nickel and its alloys and German silver, consisting initially of about. 15% of sulfuric acid and 63% of ortho phosphoric acid, both by weight of said solution the balance being mainly water.

4. An electrolytic solution for use in the anodic polishing of metals selected from the group consisting of iron and its alloys, nickel and its alloys and German silver, consisting initially of 15m of sulfuric acid, 63 to 67% of ortho phosphoric acid, both by weight of said solution, and the balance being mainly water.

5. The method of electrolytically polishing a metal selected from the group consisting of iron and its alloys, nickel and its alloys and German silver, which comprises making the metal the anode in an aqueous solution consisting essentially of from 5 to 85% sulfuric and from 5 to 85% ortho phosphoric acids having a combined acid content in excess of 50% but not over 90% by weight of said solution the balance being mainly water and passing an electric current therethrough of sufficient density and for a sufficient length of time to produce a polish on said metal.

6. The method of electrolytically polishing a metal selected from the group consisting of iron and its alloys, nickel and its alloys and German silver, which comprises making the metal the anode in an aqueous solution consisting initially of from 15 to 60% of sulfuric and from 15 to 70% of ortho phosphoric acids having a combined acid content in excess of but not over 90% by weight of said solution the balance being mainly water and passing an electric current therethrough of sufficient density and for a suflicient length of time to produce a polish on said metal.

7. The method of electrolytically polishing a metal selected from the group consisting of iron and its alloys, nickel and its alloys and German silver, which comprises making the metal the anode in an aqueous solution consisting initially of about 15 to 20% of sulfuric and about 63 to 67% of ortho phosphoric acids by weight of said solution and passing an electric current therethrough of sufficient density and for a sumcient length of time to produces polish on said metal.

CHARLES L. FAUST. 

